A cast-coated paper is generally obtained by applying a coating solution containing a pigment and an aqueous binder to at least one side of a raw paper and pressing the resultant coating to a heated specular metal surface while it is in a wet state to copy the specular metal surface on the coating and at the same time to dry the coated paper. The cast-coated paper thus obtained has a feature of high gloss and high smoothness, compared with general coated papers which are finished with a super calender. Accordingly, it gives a very excellent printing effect when used in printing and a high-quality feeling when used for packaging material.
Further, in the field of full color image recording paper and the like whose markets are developing now, recording paper having high gloss has been produced by using a film as substrate or by adopting a wet-type lamination technique or a multilayer coating technique. In comparison with those production methods, a cast coating method is favorable in terms of cost and simplicity of processes. However, it has s drawback of being inferior in productivity to the method for producing a general coated paper.
More specifically, in the cast coating method, a coated paper must to be dried while the wet coating is pressed to the specular surface of metal. Consequently, the water contained in the coating should pass through a raw paper and evaporate on the back side of the paper. In cases where a general coated paper is produced, on the other hand, water is evaporated on the front side or both sides of the coating. Therefore, drying efficiency in the cast coating method is far lower than that in the preparation of a general coated paper; as a result, a cast-coated paper should be produced at a low speed, and thereby the productivity thereof becomes low.
For the purpose of improving drying efficiency in the cast coating method, it was proposed to increase the permeability of a raw paper in, e.g., Japanese Tokkai Sho 56-26094 and Japanese Tokkai Hei 3-193995 (The term "Tokkai" as used herein means an "unexamined published patent application"). However, the coatings of the cast-coated papers disclosed therein have a high coverage rate of from 10 to 30 g/m.sup.2, so that a lot of water to be evaporated is present therein. On this account, the aforementioned means cannot sufficiently improve the productivity.
Further, the inferiority of the cast coating method in productivity is also due to low capability in continuous operations.
More specifically, in the production of a cast-coated paper, the releasability of paper web from a specular metal surface gradually deteriorates with the passage of time; as a result, the coating is partially picked off by the metal surface, namely the so-called "drum pick" occurs.
When the drum pick occurs, the paper web is broken in a short time and at the same time the specular metal surface becomes dirty in most cases. Consequently, the operation is interrupted for a long period of time for cleaning the specular metal surface.
In this connection, the specular surface of metal has so far been coated with a release agent, such as vegetable hardened oil, vegetable oxidized oil, tributyl phosphate, xylene, terpene, monochlorobenzene and their derivatives, to maintain good releasability of the specular metal surface from paper web.
Further, there has been added a release agent, such as stearic acid, polyethylene, polypropylene, paraffin, silicone oil and their derivatives, to a coating solution, a coagulating solution or a re-wetting solution to obtain good releasability of the paper web.
However, even when the above treatments for enhancing the releasability are carried out, the releasability of paper and that of specular metal surface are gradually deteriorated with a lapse of time.
Reasons therefor are considered as follows:
a) A release agent applied to the specular surface of metal gradually shifts into the coating to lose its effect, and PA1 b) A slight portion of the coating remains on a specular metal surface every time the paper web is released from the specular surface, and its accumulation on the specular surface makes it impossible to copy the specular surface and at the same time to lose the releasability from each other.
In an ink-jet recording system, on the other hand, images are formed on a recording paper by jetting droplets of ink in various ways, and such a recording system has features that it makes less noisy than dot impact recording systems and enables easy full color recording and high-speed recording.
Usually, an ink-jet recording system uses a water-base ink in most cases, and so it has a disadvantage of being inferior in drying properties of ink.
Therefore, it is required for the recording paper used for ink-jet recording system to have the following characteristics:
a) high drying speed of ink, PA0 b) neither feathering nor overflow of ink, PA0 c) high recording density, and PA0 d) no cockle generation due to absorption of ink.
It has already been proposed to produce ink-jet recording papers having all of those characteristics using a cast coating method (Japanese Tokkai Sho 62-95258, Japanese Tokkai Sho 62-264391, Japanese Tokkai Hei 2-274587 and Japanese Tokkai Hei 5-59684).
However, the cast-coated papers for ink-jet recording contain as the main pigment component a porous synthetic silica of large specific surface area, so that they absorb a great amount of release accelerating material to cause frequent paper break, thereby suffering a great drop in productivity, compared with cast-coated papers using other pigments as their main pigment component.